This invention relates to an improved process for preparing a liquid anhydrous 3-chloro-2-sulfopropionic acid reagent from acrylic acid and chlorosulfonic acid. It also relates to a novel process for preparing an aqueous reagent which is easily pourable at temperatures below 25.degree. C. It further relates to the isolation of 3-chloro-2-sulfopropionic acid in crystalline form.
The technical literature reveals two basic processes for preparing 3-chloro-2-sulfopropionic acid reagents. One involves the sulfonation of 3-chloropropionic acid with sulfur trioxide (see Bull. Soc. Chim. Fr. (7-8), Part 2, 2266, 1973); the other involves the reaction of acrylic acid and chlorosulfonic acid (see Bull. Soc. Chim. Fr. cited above and U.S. Pat. No. 2,895,987).
The sulfonation process requires the addition, with cooling, of 3-chloropropionic acid to liquid sulfur trioxide. The mixture is then homogenized, heated to 80.degree. C. and maintained at 80.degree. C. for 24 hours, and cooled. The resulting syrupy mass contains about 90% 3-chloro-2sulfopropionic acid, 3.5% unreacted 3-chloropropionic acid, and 6.5% sulfur trioxide. This process suffers from several drawbacks, namely one of the reagents is relatively expensive (i.e. 3-chloropropionic acid) and the other is difficult to handle (i.e. sulfur trioxide). Also it requires reagent addition at low temperatures, homogenization, and slow heating to the final reaction temperature.
The process described in the above cited U.S. patent reveals the 3-chloro-2-sulfopropionic acid as an intermediate which is not isolated. A half mole of chlorosulfonic acid is added dropwise to acrylic acid, the dropwise addition being necessary since rapid addition causes the temperature to rise to about 145.degree. C. The mixture is then cooled to 80.degree. C., the remaining chlorosulfonic acid is added at this temperature over about 10-15 minutes, and the mixture is stirred for 2 hours at 65.degree. to 90.degree. C. and then cooled.
The process wherein chlorosulfonic acid is added to acrylic acid is a potentially very dangerous process for large scale reactions. The reagent addition is an extremely exothermic reaction, and it requires cooling and very slow chlorosulfonic acid addition, followed by slow heating to the reaction temperature (80.degree. C.). If the temperature becomes too high during the addition or if the reaction mixture is heated too rapidly to 80.degree. C., the mixture rapidly increases in temperature (to above 150.degree. C.) and the final product can form as an insoluble rubbery mass (probably polyacrylic acid).
The other process described in the French journal involves the addition of acrylic acid to chlorosulfonic acid maintained at 0.degree. to 10.degree. C. The mixture is then heated slowly to 80.degree. C., maintained at that temperature for 20 hours, and cooled. The resulting reagent contains about 90% 3-chloro-2-sulfopropionic acid, 2% 3-chloropropionic acid, and 8% chlorosulfonic acid.
The process wherein acrylic acid is added to chlorosulfonic acid has several drawbacks, namely the low temperature addition (0.degree.-10.degree. C.) and long reaction time (20 hours), but more importantly it too requires slow heating to 80.degree. C. to avoid the dangerous exotherm.
In addition, the reagent resulting from either of the latter two processes is not completely satisfactory. It is a brown liquid, relatively thin at temperatures above 35.degree. C., very heavy but pourable at 25.degree. C., but extremely viscous and not readily pourable at lower temperatures. It contains unreacted chlorosulfonic acid, and the low vapor pressure of this unreacted acid leads to acidic fuming when the reagent is poured. A further handling problem could develop during winter time storage at low temperatures, when the reagent could not be pumped or poured without heating the storage drums.
Hence, the search has begun for a safer and quicker process for preparing the anhydrous 3-chloro-2-sulfopropionic acid reagent and for preparing an aqueous reagent which is free of unreacted chlorosulfonic acid and pourable even at low temperatures and for a purer form of 3-chloro-2-sulfopropionic acid.